The Therapeutic Index of a drug is the ratio of the toxic to the therapeutic dose. Drugs with a low therapeutic index may only require a small increase in dose to produce toxic effects. Drugs with a low therapeutic index are often dosed after consideration of relevant parameters such as weight, body mass index (BMI), other drug therapy, renal function, or a monitoring variable such as INR or drug levels in the plasma. Some drugs have differential metabolism due to individual genetics and sometimes this can be predicted as with azathioprine and thiopurine methyltransferase (TPMT) activity.
- digoxin and digitalis glycosides
- warfarin and other coumarin anticoagulants (eg acenocoumarol, phenindione)
- phenytoin due to its satuation pharmacokinetics
- chemotherapeutic agents for malignancy and immunosuppression
- paracetamol...subsets of patients have enhanced toxicity (ie low therapeutic index)